1. Field
This disclosure relates generally to communication systems and equipment, and more specifically, to techniques and apparatus for echo cancellation or echo reduction in a communication system.
2. Related Art
Echo has a major effect on voice quality in telecommunication networks (such as the Public Switching Telephone Network (PSTN) or Packet Telephony (PT) network). The objectionable effect of echo results from a combination of reflections from network components such as two- to four-wire converters (e.g., an impedance mismatch of a hybrid circuit, which is a device used to convert signals from a four-wire communication network interface to a two-wire local subscriber loop, and vice versa), together with signal processing and transmission delay. Echo may cause users difficulty in talking or listening over a telephone connection, and it may also affect the transmission of voiceband data, fax and text.
Echo cancellation is used in a telecommunications network to ensure voice quality through elimination or reduction of electric or line echo from the telecommunications network. Echoes develop, or are created, in an “echo path,” which is a circuit that includes all transmission facilities and equipment (including the hybrid and terminating telephone set) connected to the near end of an echo canceller.
An echo canceller is a device that can use adaptive signal processing to reduce or eliminate echoes to allow successful transmission of voice and/or voiceband data (such as modem and facsimile signals). Echo cancelers can be placed in the four-wire portion of a circuit, and reduce (or cancel) the echo by subtracting an estimate of the echo from the returned echo signal. For a more detailed discussion of echo cancelers, see the document entitled “Digital Network Echo Cancelers,” which is published by The International Telecommunication Union's (ITU) Telecommunication Standardization Sector (ITU-T) as ITU-T Recommendation G.168.
One characteristic of a good echo canceller is rapid convergence. Convergence can be generally defined as the time the echo canceller needs to produce an estimate of an echo signal and reduce the echo signal below a threshold. Adaptive signal processing circuits may be used in an echo canceller to remove or facilitate removal of the echo signal. The adaptive signal processing circuits unfortunately may increase the complexity and computational costs associated with rapid and accurate echo signal identification and cancellation or removal.